Professor lindström in a recent paper has described sense organs discovered by him on the hypostome of very many genera of Trilobites. His description is as follows:— “In the plurality of species there are two tiny patches or maculæ, sometimes elevated above the surrounding surface like tubercles, and so they have also been called by some authors. But I have preferred to use the name ‘macula’ for them, as the plurality does not form tubercles. They are generally smooth and glossy, and situated next to the anterior groove, either above it or in it, at a regular distance from each other and the lateral margins. They may form a sunk spot, or, as commonly, an ovoid or elliptic area surrounded by a linear elevated border.” “Common for a great number of maculæ in various groups, whether they show any organic structure or not, is the excessive thinness of their shell in comparison with that of the surrounding hypostoma. This is also in accordance with the tenuity of the cephalic eyes in relation to the test of the cheeks.”